1. Field of the Invention
This invention relates to a method of and a device for simultaneous crystal diameter measurement to be used in an apparatus for automatically controlling single crystal growth by the Czochralski technique.
2. Description of the State of the Art
A basic description of the use of observations by a single visual camera of the meniscus of the solid-liquid interface in growing a silicon single crystal by the Czochralski technique in order to monitor and control the diameter of the resulting crystal is given by Diggs et al. in their article, The Basis of Automatic Diameter Control Utilizing "Bright Ring" Meniscus reflection, in Journal of Crystal Growth, 29:326-28 (1975).
The use of a laser, as opposed to an optical camera, to measure the diameter of a growing crystal was described in Monitoring Diameter Variation and Diameter Control Using Laser Beam and Image Processing Czochralski Crystal Growth, IBM Technical Disclosure Bulletin, Vol. 27, No. 10A, pages 5777-5778 (1985). This document also gives a basic description of control methodology for automatically changing growth parameters when a diameter deviation occurs to bring the diameter back to its target value.
Baba et al., U.S. Pat. No. 5,138,179, discloses using a single camera for diameter measurement on growth of neck section of a crystal and describes the interaction between the optical camera and automated crystal growth controls.
Bonora, U.S. Pat. No. 3,998,598 discusses the role of melt level in measuring diameter with an optical pyrometer and indicates that moving the melt level while growing results in unwanted flats in the crystal. Bonora thus teaches to adjust the angle of the pyrometer or camera and the mathematical formulas used to calculate diameter in order to maintain optical diameter accuracy as the melt level is reduced.
Katsuoka et al., U.S. Pat. No. 4,915,775, discloses a technique for optically calibrating the melt level location before crystal growing commences and then maintaining the melt level through mathematical algorithms throughout the crystal growth process.
Scholl et al., U.S. Pat. No. 4,350,557, discloses a method of attaching a sensor to the pulling cable that triggers a receiver, allowing the machine to determine each time the crystal has made a complete revolution, so that a radiation sensitive controller can then measure an integrated radiation signal around the circumference of the crystal at the melt meniscus.
Fuerhoff U.S. Pat. Nos. 5,653,799 and 5,656,078 disclose a method for controlling growth of a silicon crystal and a video camera for use in a system for controlling growth of a silicon crystal. The melt from which the crystal is pulled has a surface with a meniscus which is visible as a bright ring adjacent the crystal. A video camera generates an image pattern of a portion of the bright ring, and image processing circuitry defines an edge of the bright ring and a generally circular shape including this defined edge. The diameter of the crystal is then determined from the defined circular shape.
A conventional method of crystal diameter measurement, as illustrated in FIG. 1, shows a luminous ring 74, which is formed at the interface between the surface of a silicon melt 42 and a single silicon crystal 40 being pulled out of the melt. This luminous ring 74 is photographed by a single CCD camera, and the resulting video signal is binary-coded to obtain a binary image contained in an automatic control system. A particular portion such as in inner diameter D.sub.i 70, or the outer diameter D.sub.o 72, contained in this binary image is detected and multiplied by precalculated constants to determine the diameter of the crystal being grown. This diameter, however, is calculated with the assumption that the center of the crystal is at a known place, and as such the uses the location of the optically detected luminous ring 74 to calculate the distance from the detected point on the luminous ring 74 to the theoretical center of the ingot to calculate a radius, which is then converted to a diameter.
Although generally effective, there are several issues that can effect the accuracy of the diameter calculations. One key factor encompasses a phenomenon known as orbit. As the crystal is being grown it is rotated around its longitudinal axis at a controlled rate. If the mass of the crystal or the various mechanical parts used to hold and raise the crystal from the melt is not perfectly centered along the longitudinal axis of the crystal, the entire crystal will swing in a circular motion. This swinging motion is known as orbit. Obviously, as the crystal orbits in the melt, the luminous ring 74 moves, causing the camera to incorrectly perceive that the diameter of the crystal is changing rather than sensing orbit. The automated control system then tries to compensate for the incorrectly perceived diameter change by adjusting temperature and/or pulling speed, resulting in an actual loss of diameter control.
Another phenomenon that can affect the diameter calculations when using the above described conventional method is the location of the melt level in the crucible.
Camera angle can also play a critical role in diameter measurement accuracy. The camera is "calibrated" to a known specific angle to the viewing area. This known angle is used in the mathematical algorithms for calculating ingot diameter. If the camera is bumped or moved after calibration, the theoretical calculations are incorrect due to the predicted sensing location and the actual sensing location not being the same.
Using a prior art system as described above, the total diameter error is then the sum of the orbit error, twice the melt level error, and the camera angle change error. Although it is possible to have the errors offset each other, such as having one error indicate a smaller than actual diameter while another indicates a larger than actual diameter, it is unfortunately also possible to have all errors indicate diameter that is either too small or too large. This compounding of errors is a worst case scenario, and can result in significant material loss due to oversized, or undersized crystals. Thus there has remained a need in the art for a more accurate and reliable method and apparatus for measuring and controlling the diameter of a silicon single crystal grown by the Czochralski technique.